Improving the efficiency of any air consuming process like combustion, air separation, air compression for obtaining mechanical power is of considerable importance as this leads to saving of the fuel used for these processes. Most of the common fuels consumed are non replenish-able hydrocarbons. Inefficient burning of these fuels results in extensive pollution and is one of the main causes of global warming. If the air to fuel ratio is correctly maintained then the fuel is fully burned and causes little or no pollution to the atmosphere. Reducing the amount of fuel is one of the ways of improving the combustion process but leads to severe drop in power produced and is not feasible beyond certain levels. Improving the quality and amount of air available for combustion process is a better option.
Air required for any combustion process needs to be free of dust particles as the presence of dust particles cause damage to fuel injection nozzles, cooling slits in the combustion chamber and other parts of the fuel metering system. Therefore air is filtered by introducing a proper filtration system in the inlet air duct. Introduction of a filtration system in the inlet duct causes pressure drop and causes reduction of air mass passing through the filter. Over a period of use, the filters tend to get choked with entrapped dust particles causing further pressure drop across the filter and proportionate reduction in the inlet air mass due to reduction of inlet air pressure.
Compressing the inlet air is one of the methods of improving the mass of air available for the combustion process. A reciprocating or rotary compressor is used after pre-filtration to compress the air to increase the air density and thereby improve the air mass available for combustion. Compressing the inlet air increases the air temperature above the ambient. In temperate regions where the ambient temperatures are low, the increase in inlet air temperature due to compression is of no significance. Whereas in tropical and arid conditions the ambient temperature of the air is in the range of 15° to 30° Celcius, further rise in air temperature due to the compression process reduces the overall efficiency of the combustion process.
The efficiency of a gas turbine, or more correctly the overall thermal efficiency, is the ratio of mechanical work done to the heat supplied. The Carnot efficiency is defined as:ηcarnot=W/Q=(Tmax−Tmin)/Tmax Where:W=mechanical work heat supplied Q=heat supplied Tmax=maximum temperature Tmin=minimum temperature
With the Carnot formula, efficiency can be expressed as temperatures. For gas turbines, Tmax is the temperature of the hot gases leaving the combustion chamber gases and Tmin is the ambient temperature.
If it is assumed that fuel is burnt at stoichiometric conditions to obtain a flame temperature of 2500 K and a turbine is designed to withstand 2500 K, then the engine is working at a pressure ratio of 100, which is very high but not unreasonable assuming that the internal losses are negligible.
Based on above, such a hypothetical gas turbine results in an efficiency of approximately 65%.
However, in comparison, the best-performing, simple, open-cycle, single-shaft machines show an efficiency of approximately 40%.
From the Carnot efficiency formula above, it is found that efficiency can be increased by increasing the temperature span between the heat source and heat sink. The heat sink cannot be made to go to a lower temperature, according to the second law, but the heat-source temperature can be increased, for example by firing more, fuel. Firing more fuel raises the temperature and increases the temperature span.
The difficulty in increasing the firing temperature is to find durable materials for use in combustor linings and turbine blades. When new materials become difficult to find, then other ways have to be found to get around the problem of overheating. Another method is to enhance the mass of air being fed to the air consuming process.
Theory that forms the basis of this invention is based on the fact that gas turbines ingest a constant volume of air regardless of the ambient air temperature. The gas turbine's power output increases as air mass flow rate increases provided other variables are kept constant. With the constant volumetric flow of a gas turbine, by increasing the air compressor inlet air density, more mass flow rate is achieved. The power produced by the turbine is nearly a linear function of air mass flow rate.
It is therefore conceivable that increased mass flow can be achieved by increasing the air density. If the additional mass flow from the fuel is ignored, then the ideal gas equation in respect to mass flow rate is:m=P1V1/RT1
This equation suggests that the power output is a linear function of air density and linear inverse function of temperature.
Various attempts have been made to resolve the problems encountered in enhancement of process air inlet mass. Some of the patents are listed below: